Metal cored electrode

ABSTRACT

A metal cored electrode for CO 2  gas shielded welding has 5% to 30% core ingredients in a ferrous metal tube with a seam. The core includes, based upon the weight of the electrode: from about 0.01% to about 0.5% cesium, from about 0.1 to about 3.0 cesium/other alkali metal(s) on a weight basis, the other alkali metals selected from the group consisting of lithium, sodium, potassium and rubidium; and 
     from about 0.3% to about 10% of alloying elements selected from silicon, manganese, and optionally titanium, columbium, aluminum, nickel, chromium and cobalt; 
     up to 0.08% boron; and 
     balance iron together with incidental impurities. 
     The electrodes are used in reverse and straight polarity DC and AC CO 2  gas shielded welding processes for providing stable arcs and reduced spatter levels as well as high quality weld deposits.

BACKGROUND OF THE INVENTION

1. Statement of Related Application

This is a division of co-pending application Ser. No. 425,725, filed onOct. 23, 1989, now U.S. Pat. No. 4,999,478.

2. Field of the Invention

This invention relates to a metal cored electrode for CO₂ gas shieldedmetal arc welding, and more particularly to a cesium-containing seamedmetal cored electrode for reverse polarity welding.

DESCRIPTION OF THE PRIOR ART

Relatively expensive inert gases such as argon and helium have long beenemployed to shield welding arcs from the atmosphere for obtaining highquality welds in reverse polarity as well as straight polarity weldingand in A.C. welding processes. The art much prefers reverse polaritywelding (where the workpiece is negative) because a negative workpieceis hotter than a positive workpiece which permits better penetration.See, e.g., U.S. Pat. Nos. 2,806,128, 2,932,722 and 3,147,362 and thepatents cited therein; and also Cushman, "Electrode for Spatter-FreeWelding of Steel in Carbon Dioxide," Welding Research Supplement, Jan.1961 at pages 14-s to 21-s. As this art discloses in some detail, CO₂-containing gases are employed as shielding gases in place of inertgases to reduce the cost of gas shielded welding. Such gases generallyinclude 25% or more CO₂ and 75% or less inert gases. However, metaltransfer through CO₂ shielded arcs tends to be globular in form and tobe particularly hard to control in reverse welding processes. Thisresults in unstable arcs and excessive spatter levels which leads to lowquality welds, reduced weld efficiency and high clean-up costs.

U.S. Pat. No. 2,932,722 discloses an improved electrode which provides acoating containing so-called emissive agents including cesium (carbonateor hydroxide) in combination with other alkali metals to develop a sprayform of metal transfer and thereby to reduce the spatter emissions.However, as disclosed in U.S. Pat. No. 2,932,722, welding withcesium-coated electrodes can only be controlled when operating withstraight polarity (where the electrode is negative and the work piece ispositive).

U.S. Pat. No. 3,147,362 discloses that the cesium-containing coating ofthe electrode of U.S. Pat. No. 2,932,722 is so hygroscopic that thecesium compounds tend to break down and that surface films tend tocreate electrical contact problems. Thus, this patent proposes toprovide emissive materials in a fill. The core materials, includingmetal scraps coated with alkali metal carbonates (comprising less than1% cesium carbonate and more than 99% rubidium, potassium, sodium andlithium carbonate), are compacted in a metal tube and the filled tube isthen extruded and drawn. According to this improvement, the cesium andother highly hygroscopic ingredients in the core are sealed againstatmospheric moisture and there are no electrical contact problemsexperienced of cesium-containing coatings.

The metal cored electrode disclosed by U.S. Pat. No. 3,147,362 has notreceived commercial acceptance because commercial composite electrodesare preferably not made by filling tubes. Rather, flat metal strips arecontinuously formed into U-shaped configurations, filled with coreingredients, and then bent into tubes having seams defined by adjacentedges of the strips and finally drawn to size. It is very difficult tocontinuously and accurately feed cesium containing compounds into suchstrips because cesium readily absorbs moisture from the atmosphere andthen forms clumps which begin to plug the equipment. In addition,moisture from the atmosphere is unavoidably drawn through the seams ofcomposite electrodes into the core, which may then result in high levelsof diffusible hydrogen in weld deposits. Desirably, electrodes deposithigh quality welds with less than 5 ml hydrogen per gram of depositedweld metal (preferably by reverse polarity CO₂ gas shielded weldingprocesses) even after having been exposed to moisture for several dayswhile, e.g., laying on a shelf.

SUMMARY OF THE INVENTION

The present invention provides a seamed metal cored electrode containingsmall amounts of cesium and other alkali metals for CO₂ gas shieldedwelding. Spray type metal transfer, stable arcs and reduced spatteringare obtained during reversed polarity as well as straight polarity D.C.and A.C. welding not withstanding moisture pickup which inevitablyoccurs during the fabrication process and later through the seam of theelectrode. The ingredients in the core of the metal tube of theelectrode comprise, by weight percent of the metal cored electrode, from0.01% to 0.5% cesium and at least one other alkali metal selected fromthe group consisting of lithium, sodium, potassium, and rubidium in aweight ratio of from about 0.1 to about 3.0 cesium/other alkali metal,from about 0.3% to about 10% of at least one metallic alloying element,up to about 0.08% of boron and the balance being iron and incidentalimpurities.

At least about 0.01% cesium is needed to effectively stabilize thewelding arc and more than about 0.5% cesium tends to make the weldingarc sluggish. Preferably, the electrode contains from 0.07% to 0.2%cesium for obtaining very low spatter levels. The other alkali metalingredient is preferably comprised mainly of sodium because electrodesembodying the present invention comprising cesium and sodium emissivecompounds have significantly improved arc stability and reduced spatterlevels in reverse polarity welding processes compared with electrodesembodying the present invention containing potassium or lithiumcompounds in place of sodium compounds. Most preferably, electrodes ofthe present invention contain about 0.04% to about 0.17% sodium inaddition to the cesium for developing spray metal transfer.

Common metallic alloying elements including 0.2% to 2% silicon, 0.1% to3% manganese, and optionally up to 0.5% titanium, up to 0.5% columbium,up to 0.5% aluminum, up to 3% nickel, up to 3% chromium and up to 3%cobalt may be present in total amounts of from about 0.3% to about 10%for obtaining the desired properties in the weld deposit. They may beprovided as an alloying ingredient or in the cesium source as ainclusion.

The boron may be added as a microalloying ingredient in accordance withthe disclosure of U.S. Pat. No. 4,282,420, which is hereby incorporatedby reference.

The balance of the core ingredients comprises iron together withincidental impurities, including the type of impurities routinelycontained in metals such as carbon, oxygen, nitrogen, sulfur,phosphorous and the like and inclusions normally contained in metalcored electrodes such as the anions and cations with the alkali metals(which are provided in the form of salts).

In a preferred embodiment of the invention, the electrode contains lessthan about 1% moisture in its core, based upon the weight of the tubeand the core materials after exposure to 80% relative humidity at 37° C.(80° F.) for nine days. Electrodes, embodying the present invention,having less than about 1% moisture in their core with the coreingredients, effectively promote stable welding arcs and reduce spatter.Preferably the electrode contains less than about 0.5% moisture, andmost preferably, less than 0.3% moisture for depositing quality weldshaving less than 5 ml hydrogen/gram weld metal deposit, no porosity orother unacceptable defects.

Other details, objects and advantages of the invention including methodsof practicing it will become apparent as the following description ofpresently preferred embodiments thereof proceeds.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings present results comparative tests conducted onelectrodes embodying the present invention wherein:

FIG. 1 generally shows the effect of the cesium/alkali metal ratio onspatter levels of welds deposited by a presently preferred electrode;

FIG. 2 generally shows the effect of the cesium/alkali metal ratio onamp variations; and

FIG. 3 generally shows the effect of the cesium/alkali metal ratio onvoltage variations.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Exemplary electrodes for CO₂ gas shielded welding embodying the presentinvention generally have seamed ferrous metal tubes with coreingredients (based on their weight alone) which include: 0.5% to 1%cesium chromate; 0.5% to 1.5% sodium carbonate; 18% to 26% ferroalloysof silicon, manganese and titanium, in order to provide 0.2% to 2%silicon, 0.1% to 3% manganese and up to 0.5% titanium (based on theelectrode weight); and the balance comprising iron powder together withincidental impurities, mainly carbon. The core ingredients normallycomprise from about 5% to about 30% of the total electrode weight. Thetube which may be from about 1 mm or less up to about 3 mm or more indiameter, may be comprised of carbon, stainless or alloy steel.

Electrodes having seamed 1.2 mm (0.045 inch) diameter mild steel tubesand about 17% of core ingredients present within the above limits weretested against a comparative electrode without cesium-containing saltsin a gas shielded reverse polarity welding process using a DC powersource at about 260 amps, 29 volt. The shielding gas was nominally 100%CO₂. Six inch long welds were deposited in a flat position and thespatter emissions were collected and weighed. The spatter level was thendetermined per kilogram of weld metal deposited.

FIGS. 1 to 3 generally present the results of the comparative tests. Theencircled data point indicates the results of the tests conducted withan electrode which did not contain cesium. FIG. 1 shows that the spatterlevel was significantly reduced with the addition of 0.4 to 2.6cesium/sodium. Similarly, FIGS. 2 and 3 show that the amp and voltagevariations were also significantly reduced with the addition of smallamounts of cesium. These figures also show that there was a dramaticimprovement in properties over the comparative electrode by electrodescontaining about 0.8 cesium/sodium with perhaps some increase in voltageand amp variation beginning at about 2 weight percent cesium/sodium.Optimum spatter levels were obtained with electrodes comprising 1.2 to1.8 weight percent cesium/sodium, which electrodes contained 0.20%cessium/0.17% of sodium and 0.07% cesium/0.04% sodium respectively.

Similar comparative tests conducted on electrodes of this embodiment ofthe present invention and commercially available electrodes (which donot contain cesium) adapted for CO₂ gas shielded welding demonstratedthat the electrodes embodying the present invention deposited a flatbead profile in horizontal fillet welds at higher wire speeds and travelspeeds than did the comparative commercial electrodes. The commercialelectrodes produced arcs that were buried by molten puddle when weldingat high wire speeds, which subjects the deposited weld metal to thedanger of lack of fusion or slag entrapment.

Cesium and the other alkali metals may be provided in any suitable formin addition to chromate salts. Thus they may be provided as tartrates,permanganates, titanates, aluminates, stearates, carbonates,columbiates, tantalates, zirconates and like compounds, which may alsoinclude iron, nickel, chromium, calcium and the like. Cesium chromate isa preferred cesium source. Electrodes containing cesium chromate as acesium source were exposed to 80% relative humidity/55° C. (130° F.) for120 hours. Those electrodes deposited high quality welds having noporosity or other detectable defect in a CO₂ gas shielded welding test.

Cesium titanate is another preferred cesium source. Leucoxene and othernatural titanate sources may advantageously include such metallicalloying elements as chromium and columbium, but also impurities such ascalcium and fluorine. Whatever the source of cesium and the otheringredients, electrodes embodying the present invention contain up to 3%chromium, up to 0.5% titanium and up to 0.5% columbium. Preferably, theelectrode contains no more than 0.06% titanium and more preferably nomore than 0.02% titanium for good spray transfer. Also the electrodepreferably contains no more than 0.1% columbium and more preferably nomore than 0.05% columbium for good spray transfer. The alkali metals mayalso be provided as halogen salts, but these salts are not preferredbecause they are extremely hygroscopic. Thus, they are particularlydifficult to transport and feed through weighing equipment whilemanufacturing electrodes. Such hygroscopic salts may be agglomerated bybaking a water glass film on the particles to facilitate their use asalkali metal sources. Preferably, the other alkali metals in addition tocesium includes no more than trace amounts of rubidium which is highlyhygroscopic.

In addition to single salts, cesium may be provided as a double saltfrom natural sources or synthetically. Thus, e.g., cesium aluminumsilicate may be provided from pollucite. Alternatively, and more costly,double salts such as cesium aluminum columbiate may be synthesized bysol-gel or other processes. Whatever may be the source of cesium andother ingredients, electrodes embodying the present invention contain nomore than 0.5% aluminum, which tends to form refractory aluminum oxideson the metal being transferred. Preferably, the electrodes contain lessthan 0.1% aluminum for minimizing gobular metal transfer across thewelding arc and more preferably less than 0.01% aluminum for obtainingspray transfer across the arc. FIG. 1 includes two data pointsidentified by carets which summarize the results of spatter tests onmetal cored electrodes generally similar to the above identifiedelectrodes of the test, but containing cesium in the form of a cesiumaluminum columbiate double salt. Thus, FIG. 1 indicates that doublesalts containing aluminum and other refractory elements such ascolumbium or tantalum may also be useful sources of cesium in thepresent invention. However, double salts including halogen salts such ascesium fluorotitanate or the like are not preferred because halogensalts are generally hygroscopic.

The alloying ingredients are preferably provided with the alkalimetal-containing compounds and/or as alloys including ferroalloys, butthe impurities associated with them should not substantially adverselyaffect the welding process or the deposited weld metal. The iron isnormally in powder form.

Preferably the electrode contains less than about 1% by total electrodeweight moisture after exposure to 80° relative humidity at 35° C. (80°F.) for nine days for reducing the effect of moisture upon the cesiumduring the welding process and the deposited weld metal. Morepreferably, the electrode contains less than 0.5% moisture for providingan extended shelf life. Electrodes containing less than 0.3% moistureexposed to highly humid atmospheres have produced high quality welds.

Additional CO₂ gas shielded welding tests were performed with otherelectrodes having a 17% fill which had in their cores alloyingingredients including up to 6% silicon, up to 12% manganese and up to 1%titanium, at least about 70% iron and emissive agents in the followingamounts (based on the weight of the core ingredients alone):

    ______________________________________                                        Electrode                                                                     Ingredient  A       B           C    D                                        ______________________________________                                        Cs.sub.2 TiO.sub.2                                                                        --      --          --   1.40                                     Cs.sub.2 CO.sub.3                                                                         0.5-1.0 0.5-1       1.0  -                                        Na.sub.2 CO.sub.3                                                                         --      --          1.25 1.25                                     K.sub.2 CO.sub.3                                                                          --      0.4-0.7     .3   --                                       Li.sub.2 CO.sub.3                                                                         0.5     --          --   --                                       ______________________________________                                    

These electrodes had improved arc stability, and reduced spatter levelsas well as flatter bead shapes and less roll-over compared withcommercially available electrodes not containing cesium.

While certain presently preferred embodiments of the present inventionhad been described, it is to be distinctly understood that the inventionis not limited thereto, but may be variously embodied within the scopeof the following claims.

What is claimed is:
 1. A metal cored electrode for CO₂ gas shielded welding having core ingredients in a ferrous metal tube.the metal tube having a seam; and the core ingredients including (by weight of the electrode):(a) from 0.01% to 0.5% cesium; (b) at least one other alkali metal selected from the group consisting of lithium, sodium, potassium and rubidium in a weight ratio of from about 0.1 to about 3.0 cesium/other alkali metal(s); (c) from 0.3% to 10% of at least one metallic alloying element selected from the group consisting offrom 0 2% to 2% silicon, from 0.1% to 3% manganese, up to 0.5% titanium, up to 0.5% columbium, up to 0.5% aluminum, up to 3% chromium, up to 3% nickel, and up to 3% cobalt; (d) up to 0.08% boron; and (e) the balance iron and incidental impurities.
 2. The electrode of claim 1, comprising from 0.07% to 0.2% cesium.
 3. The electrode of claim 1, wherein the other at least one alkali metal is selected from the group consisting of sodium, potassium and lithium.
 4. The electrode of claim 1, wherein the other alkali metal comprise sodium.
 5. The electrode of claim 4, wherein sodium comprises from 0.04% to 0.17%.
 6. The electrode of claim 4, comprising a weight ratio of from 0.8 to 2 cesium/sodium.
 7. The electrode of claim 4, comprising a weight ratio of from 1.2 to 1.8 cesium/sodium.
 8. The electrode of claim 1, wherein the core ingredients comprise less than 1% moisture based upon the weight of the metal cored electrode after exposure to 80% relative humidity at 35° C. (80° F.) for nine days.
 9. The electrode of claim 1, wherein the core ingredients comprise less than 0.5% moisture based on the weight of the metal cored electrode after exposure to 80% relative humidity at 35° F. (80° F.) for nine days.
 10. The electrode of claim 1, wherein the core ingredients comprise less than 0.3% moisture based on the weight of the metal cored electrode after exposure to 80% relative humidity at 35° C. (80° F.) for nine days.
 11. The electrode of claim 1, wherein the cesium is provided as cesium chromate.
 12. The electrode of claim 1, wherein the cesium is provided as cesium titanate.
 13. The electrode of claim 1, wherein the cesium is provided as cesium carbonate.
 14. A CO₂ gas shielded welding process comprising the steps of:arc welding a workpiece with a metal cored electrode of claim 1; providing a gas shield about the arc comprising at least about 25% CO₂ ; and applying reverse polarity from a power source whereby the workpiece is negative and the electrode is positive.
 15. The process of claim 14, wherein the power is supplied by an AC source.
 16. The process of claim 14, wherein the power is applied by a DC source.
 17. A CO₂ gas shielded welding process comprising the steps of:arc welding a workpiece with a metal cored electrode of claim 1; providing a gas shield about the arc comprising at least about 25% CO₂ ; and applying straight polarity from a power source whereby the workpiece is positive and the electrode is negative. 